Don't show a stack trace on boot error, do a user friendly error instead

[timja]

Jenkins and plugins versions report

What Operating System are you using (both controller, and any agents involved in the problem)?

Reproduction steps

• Put invalid config in a jenkins.yaml and restart it
• Jenkins fails to load with a big stacktrace

https://gitter.im/jenkinsci/configuration-as-code-plugin?at=61fa665aced11857f98d53c6

Expected Results

• A clear error message is displayed, possibly with the option to expand it to see the full error

Jenkins has moved away from showing stacktraces to users, and hides them by default now on error pages, we should follow to match that

Actual Results

• A giant stacktrace that scares users and makes it hard for them to see what is wrong

Anything else?

No response